Should I Sterilise My Compost?

There are mixed opinions about whether you should bother to sterilise your compost. Some gardeners choose not to, which is fine, but we believe there are many benefits to this very simple process:

It kills off harmful bacteria. Some may argue that in turn you will be killing useful bacteria but this is not the case. The only way you will kill of beneficial bacteria is by baking your soil at a temperature that is too high; we talk about this in more detail later. Professional nurseries sterilise their compost, and there’s no reason you shouldn’t either.

It’s proven to keep away pests such as thrips that are particularly annoying when using compost in your home and sterilisation can prevent such unwanted house guests.

Prevention is the solution. Prevent disease in your compost before the problems arise, rather than skipping past the sterilisation stage and then making the situation a lot worse later on.

Sterilised soil ensures that your plant will be happy and healthy, and this means the best optimal growth.

Better safe than sorry. The methods outlined in this blog are super easy to do, and will make sure your compost is definitely safe for your plants. So why wouldn’t you want to give it a go?

Outlined here are three easy methods to sterilise your compost from your home:

Oven

Using your oven at home you can sterilise your compost easily; be warned that baking compost can create a smelly odour, so you may wish to open your windows whilst doing this.

Firstly, you need to use moist soil, do not over water the soil however you only want a slight dampness.

Use an oven safe tray and fill it with your soil until it is around 10 cm (4 inches) deep.

Cover the tray loosely with foil.

Put your tray in the middle of a pre-heated oven that’s around 80° For a more accurate result use a thermometer in the centre of the tray and bake between 80-90°c

Do not exceed the temperatures stated above, at temperatures above 90°c is when the good bacteria is killed and toxins are produced.

Bake for 30 minutes before taking out, make sure to take the foil off and leave it to cool for a while before handling the soil.

Microwave

The easiest and quickest way to sterilise your compost is with your microwave. We suggest using an old microwave in your garden shed or greenhouse to prevent bringing compost into your home, and this way you can get on with other gardening jobs whilst it’s baking.

As before you will need moist soil, but not too wet that it is slushy.

Find a microwave safe container and fill this with your soil.

Do not use foil in the microwave, instead cover with cling film with holes for the steam to escape or a plastic lid with air holes.

For every two pounds of soil will need 90 seconds in the microwave.

After it’s pinged, leave the soil to cool before handling.

Alternative method:

Place two pounds of moist soil in a polypropylene bag

Leave the bag slightly open for ventilation

Zap in the microwave for 2-2 ½ minutes on full power before removing and cooling

Pressure Cooker

Start by pouring a few cups of water into the cooker

Next add your pans of soil, be careful not to add more than 4 inches, and pop it on the top rack.

Make sure to cover these with foil to help insulate the soil.

Close the lid for your cooker but make sure you leave the steam valve

For every ten pounds of soil, leave it to steam for 15-20 minutes.

Voila! You now have sterilised soil that will be sure to sprout stunning plants in no time! If you prefer shop bought compost, read our Primrose Guide to Compost for further advice and information.

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Zoë works in the Marketing team at Primrose, and is passionate about all things social media.

After travelling across Europe and Asia, Zoë is intrigued by different cultures and learning more about the world around her. If she’s not jet setting, Zoë loves nothing more than curling up with a good book and a large glass of red wine!